Add --unset option to remove environment variables

[k0kubun]

It seems like envchain currently doesn't provide a way to remove namespaces or environment variables. Sometimes you may be able to set "" as a workaround, but "" vs null matters in some applications.

So I added --unset option to remove environment variables. You can also clean up namespaces you no longer use from envchain --list if you remove all environment variables under a namespace using this feature.

Example

$ ./envchain --set test TEST1 TEST2
test.TEST1: test1
test.TEST2: test2
$ ./envchain --unset test TEST2
$ ./envchain test env | grep TEST
TEST1=test1

[gardar]

Might be a good idea to document this option in the readme?

Also perhaps a new release is due since this option only exists on master and not in any release @sorah